The invention relates to power systems and methods and, more particularly, to power systems and methods employing generators.
Power systems for facilities may serve critical, life safety and other types of loads that require high availability. A typical power system for such a facility, therefore, may include an auxiliary generator, such as a diesel-powered engine-generator set, which may supply power to these loads when a primary power source, such as a utility source, fails.
Referring to FIG. 1, a typical power system may include an input breaker 40a configured to couple a load bus 30 to a utility power source 10. Respective breakers 40d, 40e, 40f are used to couple and decouple life safety, essential and non-essential loads 80b, 80c, 80d to and from the load bus 30. Another breaker 40c is configured to couple and decouple a generator 60 to and from the load bus 30. A UPS 50 is coupled and decoupled to and from the load bus 30 by another breaker 40b, such that it may receive power from the utility power source 10 or the load bus 30 depending on the state of the input breaker 40a. In the illustrated system, the UPS 50 is an on-line UPS including an input rectifier 52 having its output coupled to an output inverter 54 by a DC link. The inverter 54 provides AC power to a critical load 80a. A battery 56 is coupled to the DC link and may supply power to the inverter 54 when the AC power delivered to the rectifier 52 fails. A bypass switch 58 may be used to bypass the rectifier 52 and inverter 54. A controller circuit 70, which may include circuitry physically distributed among the various components of the power system, controls operations of the breakers 40a-40f and the generator 60.
Typically, in response to a loss of utility power, the load bus 30 of the system is disconnected from the utility source 10 and the life safety, essential and non-essential loads 80b, 80c, 80d are disconnected from the load bus 30. The engine of the generator 60 is started and accelerated and a field voltage applied to the rotor field windings to regulate the output voltage. The generator 60 typically remains disconnected from the load bus 30 until the generator 60 achieves an output voltage and frequency within acceptable limits. Loads may be selectively connected to the load bus 30 after the generator 60 stabilizes the load bus 30.
Thus, the time required to bring such a generator fully on line may include time required to detect the outage, time required to decouple the load bus from the utility source, time to disconnect the loads from the load bus, time to bring the generator up to an acceptable voltage and frequency, time to connect the generator to the load bus and time to connect loads to the load bus. These operations may take on the order of several seconds, which may negatively affect the availability of certain loads.